The invention pertains to electronic circuits for monitoring the operation of field mounted sensors, and, in particular, circuits that annunciate the state of the sensor (i.e. on/off). Such circuits typically have circuit elements located in the field adjacent to the sensor, and have lead wires extending to a central control panel. Often, these wires are subject to electrical faults, i.e. a short circuit (usually caused by loose or sloppily installed wires), an open circuit (usually caused by a break in the lead wires), or a ground fault (usually caused by water accumulated in the field circuit box). Such faults, considered mere nuisances, are not in themselves serious threats to the process system, but do often indicate problems that can become serious (e.g. if the sensor is a thermocouple on a motor, water that shorts the detector leads could also short the motor's power supply), and in any event, render the monitor inoperative, hence depriving system operators of knowledge of the sensor's state. Thus, nuisance or not, these faults must be expeditiously corrected, usually requiring the shutting down of the system and troubleshooting the detector, an expensive and time consuming procedure.
Prior art fault detectors, for example as shown in U.S. Pat. No. 4,185,277 to the instant inventor Corso, use variations on the theme of "unanimous voting," wherein either redundant sensors, or redundant switches responsive to one such sensor, must each indicate the presence of a fault for the detector logic to announce a fault. Such an arrangement gives a system operator no indication of the kind of fault. Additionally, such a detector is highly susceptable to generating spurious fault signals due to random electromagnetic signals impinging on the sensor(s), and cannot prevent "winking" of the detector caused by power dips or by the bouncing of switch contacts, or by supply power dips.